Slips and elevators used primarily for lifting tubular goods, such as drill pipe or production tubing and the like, generally comprise a plurality of circumferentially spaced slip bodies called dies which are held collectively in a body which surrounds the locus of the drill pipe body and when used as slips the die body is in turn captured and held by a body known as a bowl. By means well known within the art, the device can be manipulated into position about the circumference of a length of pipe in a manner whereby the inner sides of the dies, having hardened metal gripping teeth, bite into and frictionally engaging the pipe body when the weight of the pipe is applied. The slip body retains the dies in place and allows the dies to have some degree of freedom with respect to the slip or elevator body, thereby allowing conformity with the pipe body. The dies are further contoured to generally conform to the curvature of the pipe body. Such slip and elevator dies are also available with various tooth configurations which help grip the pipe. Such configurations include mud grooves which allow the pipe dies to maintain a grip even in contaminated conditions, such as when the pipe is coated with mud and oil. However, it is well known in the art that damage to the pipe occurs when the dies wear unevenly or when the die teeth become damaged producing jagged edges, in which case stress risers may be set up in the surface of pipe which may result in premature pipe failure. The accepted method of gripping pipe in this manner depends on the ability of the die teeth to penetrate the surface of the pipe to some degree rather than apply excessive force which may crush or misshape the pipe.
The problem is compounded when such dies are used on high chromium pipe. Chromium or other nickel alloy pipe is often used in highly corrosive wells such as Hydrogen Sulfide (H.sub.2 s) gas wells. Such pipe is expensive and must be handled carefully to avoid damage to the chromium surface which attract corrosion, thereby leading to early failure. Therefore, a new and better means of handling such chromium and nickel alloy pipe is required in order to prevent damaging the chromium pipe surfaces. A problem also exists, when the hardened, high carbon, steel teeth on the dies make contact with the chromium or nickel alloyed pipe, thereby transferring small amounts of carbon to the pipe at each penetration point. Such carbon transfer spots have been found to set up sites for corrosion which lead to stress cracks in the pipe. It has been found that carbon creates galvanic action, thereby hardening pipe in the same manner as hydrogen sulfide, causing brittleness of the metal. Tests on chrome pipe with salt spray have shown that any discontinuity in the surface of the pipe causes a deterioration of between 0.011-0.015 loss in pipe wall thickness per year. For example, a number 13 chrome pipe having 0.217 wall thickness with a 0.028 penetration coupled with 0.015 corrosion factor per year accelerates corrosion deterioration exponentially.
Others in the art have attempted to address the problem of handling chromium pipe to and to reduce penetration, such as that disclosed by U.S. Pat. No. 5,451,084 wherein strips having hard teeth which get progressively softer along its length are held in a resilient base to allow flexibility. However such structures fail to address the problem of sharp tooth edges resulting from mud grooves cut vertically through the tooth configuration and the problem of carbon transfer to the pipe body.
Slip elevator and tong dies all rely on the biting action of the die's teeth into the pipe body for griping the pipe. However, recently the industry has begun addressing these problems by attempting to reduce stress induced into the surface of the pipe through better fits, flexible die seats, etc. However, to date, such dies still generally produce penetrations of between 0.017-0.028 of an inch with pipe loads of 14000 ft. with up to 100% carbon transfer. Test show that such high carbon deposits in the penetrations of pipe used in high corrosive wells last only a few weeks. In any case, the industry still considers die penetration of the surface of the pipe necessary. However, it is becoming essential that such penetration by the die teeth into the pipe body must be kept to a minimum, generally in the order of less than 0.002/1000 of an inch.